This invention relates to open tubular capillary columns for gas chromatography and more particularly, to stable columns lined with a polymeric material which serves as a support for the stationary phase.
The development of open tubular glass capillary columns for gas chromatography has made possible high resolution separations in a wide variety of applications. These columns can be made to almost any desired length. Therefore, a maximum number of theoretical plates may be obtained, and the resolution of any separation is enchanced. Other advantages are that only a small sample size is required and that there is a low pressure drop. Glass is an inexpensive material and is relatively inert, making it ideal for chromatographic columns. The HETP, the height equivalent to the theoretical plate, is an expression of column efficiency. It is related to the average linear gas velocity, .mu., in the van Deemter equation. EQU HETP = A + B/.sub..mu. + C.mu.
the various factors contributing to a large HETP and thus to a less efficient column are represented by A, B, and C. The term representing the effect of multiplicity of gas paths is designated as A, the term representing longitudinal diffusion as B, and the term representing mass transfer as C. Since the column is an open tube, there is only one path for the carrier gas which eliminates any diffusion due to the multiplicity of gas paths as in a packed column and therefore increases the efficiency of the column since the A term is equal to zero.
Disadvantages experienced in using glass open tubular columns are that the columns are fragile and that only a very small sample can be injected, which necessitates the use of a splitter. Of course, metal columns or metal supported glass columns are a known means of reducing the breakage problem.
This, however, does not solve the major disadvantage of open tubular columns -- their instability. This is thought to be a result of the stationary phase eventually changing from an initial thin uniform layer to individual droplets of different thicknesses and allowing the glass itself to be exposed to the solutes injected on the column. Both the exposed polar sites on the glass surface and the variable thickness of the stationary phase degrade the column performance and efficiency.
Much work has been done in the area of stabilization. One general method which has been used is the etching of the surface of the glass with hydrogen chloride gas resulting in an increase of the critical surface tension of the glass and thereby permitting the stationary phase to more easily wet the glass. See, e.g., Alexander et al, Chromatographia, Vol. 6, pp. 231+ (1973). Although this allows for the preparation of more efficient columns, it does not necessairly add to their stability. It was necessary to use soft glass for these columns since borosilicate glass does not lend itself well to etching as reported in Hishta et al., Advanc. Chromatogr., Vol. 9, pp. 215+ (1970).
Another approach, as mentioned in Jennings et al., J. Chromatogr. Sci., Vol. 12, pp. 344+ (1974) is preparation of the glass surface by rinsing with a variety of solvents in order to provide a clean surface. This has resulted in columns with useful lives of several months as well as an increased number of theoretical plates over columns which were not thoroughly cleaned. The use of chromic-sulfuric acid cleaning solution also aids in the production of a more uniform film of stationary phase by increasing the surface tension of the glass as disclosed in Necasova et al., J. Chromatogr., Vol. 79, pp. 15+ (1973). The addition of an inert solid support to the surface of the glass such as Silanox 101, a hydrophobic fumed silica dioxide, results in the formation of a thin film of stationary phase which does not break up into droplets after repeated heating and cooling. This possibility is discussed in German et al., Anal. Chem., Vol. 45, pp. 930+ (1973). Columns have also been prepared by using a colloidal suspension of graphite resulting in more efficient columns, but no report was made as to their stability. See, e.g., Nota et al., J. Chromatogr., Vol. 95, pp. 229+ (1974). Similarly, sodium chloride crystals have been used as a solid support with good results as set forth in Watanabe et al., J. Chromatogr. Sci., Vol. 13, pp. 123+ (1975), but there is no report of stability.
All of these methods are an attempt to change the surface of the glass to make it more compatible with the stationary phase. As reported in Horvath et al., Amer. Lab., August 1974 at page 75, some of these methods have met with limited success, especially with non-polar stationary phases. Polar stationary phases, however, have an even greater tendency to form microdroplets on the surface of the glass instead of remaining as a uniform film, reducing the efficiency of the column. The need, therefore, exists for the construction of stable capillary columns which can be used with polar stationary phases.